The invention described herein relates generally to structures for detecting radiation and to optical coatings for use with the structures. The invention relates more particularly to pyroelectric detectors capable of detecting visible and near-infrared fast laser pulses and to optical coatings for use with the detectors.
High speed, improved response pyroelectric sensors are used in high flux, high speed applications in the infrared and ultraviolet. The materials suitable for use in such devices need to be rugged, nonhygroscopic and sensitive. These include strontium barium niobate (hereafter SBN), lanthanum-doped lead titanate zirconate (hereafter PLZT), lithium tantalate, and lithium niobate. Other pyroelectric materials exhibit too low a sensitivity or Curie temperature and thus depole or are environmentally unstable. Unfortunately, these desirable materials are transparent and therefore unresponsive in the visible and near-infrared (ir). The ruggedness and speed of these sensors cannot be utilized for wavelengths in the 0.4 to 5.6 .mu.m region. This eliminates use with dye lasers (for most dyes), YAG, carbon monoxide, glass and other lasers which emit visible and near-ir. There are numerous applications in this region, including free electron lasers. Moreover, most pulsed laser applications in the field are in this region. Consequently, a high speed, rugged pyroelectric sensor would be highly useful.
Methods for inducing response at these wavelengths have been limited to doping, which generally affects material parameters, or applying an absorbing coating on either or both faces of the sensor. Gold black and cermets have been used. Gold black is fragile to the touch and these coatings tend to be very thick. They increase the thermal time constant of the sensor since they absorb the maximum energy, become hot and slowly release the heat into the sensor. These coatings result in slow devices which are not useful for speeds beyond about 50 kHz.
Coating with a metallic layer has not been used because of high indices of refraction and the thickness required for total absorption. A three layer coating has been applied to a PLZT detector. A. D. Annis and G. Simpson, "Absorption of Radiation in PLZT Pyroelectric Detectors," Infrared Phys., Vol. 14, pp. 199-205, 1974. This coating consists of a dielectric layer sandwiched between two metal layers. They achieved greater than 95% absorptance. The coating was, however, thick and slow.